Military and commercial communication and radar systems have a critical need for reliable, low-noise, high-efficiency, solid-state amplifiers, particularly for use in oscillator circuitry.
The performance of a microwave feedback oscillator such as a dielectric resonator oscillator (DRO) or cavity oscillator is limited by the 1/f noise of the loop amplifiers. At low frequencies (below 4 GHz), bipolar junction transistor (BJT) amplifiers have generally been employed in the oscillator circuits because of their excellent noise characteristics. However, for higher frequencies (over 4 GHz), such as in the X-band, gallium arsenide field effect transistor (GaAs FET) amplifiers have been employed, although they have poorer noise characteristics than BJT amplifiers, because of the lack of a two-port BJT amplifier. A comparison of the respective phase noise characteristics of GaAs FET and BJT amplifiers is set forth in Montress et al., "Residual Phase Noise Measurements of VHF, LIHF and Microwave Components," Proceedings of the 43rd Annual Frequency Control Symposium, IEEE Catalog No. 89CH2690-6, June 1989.
Thus, the primary reason for the utilization of GaAs FET amplifiers in these oscillators instead of BJT amplifiers is that, to date, a two-port BJT amplifier has not been developed for operation at these higher frequencies. However, as may be readily appreciated by those skilled in the art, the incorporation of BJT amplifiers in oscillators operating at higher frequencies would greatly improve their performance. Since the close-in phase-noise performance of known two-port amplifiers and oscillators currently employed in the X-band is limited by the performance of the GaAs FETs, introduction of BJTs for these higher frequency applications would result in an improvement in the system noise floor.